1. The Field of the Invention
The present invention relates generally to a spin-valve sensor for reading information signals from a magnetic medium and, in particular, to a spin-valve sensor with pinning layers comprising multiple antiferromagnetic films of varying manganese (Mn) concentrations.
2. The Relevant Art
Computer systems generally utilize auxiliary memory storage devices having magnetic media on which data can be written and from which data can be read for later uses. A direct access storage device, such as a disk drive, incorporating rotating magnetic disks is commonly used for storing data in a magnetic form on the disk surfaces. Data are recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic recording heads carrying read sensors are then used to read data from the tracks on the disk surfaces.
In high capacity disk drives, a giant magnetoresistance (GMR) head carrying a spin-valve sensor is now extensively used to read data from the tracks on the disk surfaces. This spin-valve sensor typically comprises two ferromagnetic films separated by an electrically conducting nonmagnetic film. The resistance of this spin-valve sensor varies as a function of the spin-dependent transmission of conduction electrons between the two ferromagnetic films and the accompanying spin-dependent scattering which takes place at interfaces of the ferromagnetic and nonmagnetic films.
In the spin-valve sensor, one of the ferromagnetic films, referred to as a reference (pinned) layer, typically has its magnetization pinned by exchange coupling with an antiferromagnetic film, referred to as a pinning layer. The magnetization of the other ferromagnetic film, referred to as a “sensing” or “free” layer is not fixed, however, and is free to rotate in response to signal fields from a magnetic medium. In the spin-valve sensor, the GMR effect varies as the cosine of the angle between the magnetization of the reference layer and the magnetization of the sensing layer. Recorded data can be read from the magnetic medium because the external magnetic field from the magnetic medium causes a change in the direction of magnetization in the sensing layer, which in turn causes a change in the resistance of the spin-valve sensor and a corresponding change in a sensed voltage
FIG. 1 shows a typical prior art spin-valve sensor 100 utilizing the GMR effect. The spin-valve sensor 100 is fabricated in a central region 102 separating two end regions 103 and 105. Various metallic films of the spin-valve sensor are deposited on a bottom gap layer 118, which is previously deposited on a bottom shield layer 120, which is, in turn, deposited on a substrate. Photolithographic patterning and ion milling are applied to define the central region 102 and the two end regions 103 and 105.
A ferromagnetic sensing layer 106 is separated from a ferromagnetic reference layer 110 by an electrically conducting nonmagnetic spacer layer 108. The magnetization of the reference layer 110 is fixed through exchange coupling with an antiferromagnetic pinning layer 112. This spin-valve sensor is deposited on seed layers 104, on which the sensing, spacer, reference and pinning layers of the spin-valve sensor grow with preferred crystalline textures during depositions so that desired improved GMR properties are attained. A cap layer 114 is deposited on the spin-valve sensor for protection in subsequent processes.
Longitudinal bias (LB) and conducting lead layers 126 are deposited in the end regions 103 and 105. All the metallic films deposited in the central and end regions are sandwiched between electrically insulating nonmagnetic films, one referred as a bottom gap layer 118 and the other referred as a top gap layer 124.
The disk drive industry has been engaged in an ongoing effort to fabricate a smaller spin-valve sensor for increasing the recording density of a disk drive, and correspondingly to increase the GMR coefficient of the smaller spin-valve sensor for ensuring high signal sensitivity. A higher GMR coefficients leads to higher signal sensitivity, and thus leads to a feasibility of storing more bits of information in an unit area on a disk surface, i.e., a feasibility of increasing the recording density of the disk drive. The GMR coefficient of the spin-valve sensor is expressed as ΔRG/R//, where R// is a resistance measured when the magnetizations of the sensing and reference layers are parallel to each other, and ΔRG is the maximum giant magnetoresistance (GMR) measured when the magnetizations of the sensing and reference layers are antiparallel to each other.
To ensure proper sensor operation, exchange coupling between the ferromagnetic reference layer and the antiferromagnetic pinning layer must be high enough to rigidly pin the magnetization of the reference layer in a transverse direction perpendicular to an air bearing surface. An inadequate exchange coupling may cause canting of the magnetization of the reference layer from the preferred transverse direction, thus causing malfunction of the spin-valve sensor. This ferromagnetic/antiferromagnetic exchange coupling is typically characterized by a unidirectional anisotropy field (HUA) induced from this exchange coupling. This HUA thus must be high enough to rigidly pin the magnetization of the reference layer for proper sensor operation.
A smaller spin-valve sensor operates at higher temperatures in the disk drive. The sensor operation temperature can reach as high as 180° C. and even beyond. To ensure proper sensor operation at such high temperatures in the disk drive, it is very crucial to ensure a high HUA at such high temperatures. This thermal stability is typically described by a blocking temperature (TB), where the ferromagnetic/anitferromagnetic exchange coupling diminishes and HUA is zero. A higher TB typically indicates a higher HUA at the sensor operation temperature.
The disk drive industry has thus been engaged in an ongoing effort to increase the HUA and (TB). This effort is typically devoted to the selection of ferromagnetic and antiferromagnetic films from various alloy systems as reference and pinning layers, respectively. Recently, a ferromagnetic film selected from a Co—Fe alloy system has replaced a ferromagnetic film selected from a Ni—Fe alloy system as a reference layer, in order to increase the GMR coefficient, HUA and TB. On the other hand, an antiferromagnetic film selected from a Ni—Mn or Pt—Mn alloy system as a pinning layer has been extensively implemented in the current GMR head fabrication process.
In the selection process of an antiferromagnetic film from a Ni—Mn or Pt—Mn alloy system as a pinning layer, the Mn content of the Ni—Mn or Pt—Mn film must be carefully selected. A small difference in the Mn content leads to substantial variations in both HUA and TB. In addition, since the Mn is the most diffusive and corrosive chemical element among all the chemical elements used in the spin-valve sensor, its content thus substantially determines the corrosion resistance and thermal stability of the spin-valve sensor.
The currently used Mn content of the Ni—Mn or Pt—Mn films is selected only from a small composition range for attaining a high HUA. This Mn content may not be low enough to minimize the Mn diffusion, attain a high TB, and ensure high corrosion resistance. Hence, it is difficult, or almost impossible, to find a suitable Mn content for either the Co—Fe/Ni—Mn or Co—Fe/Pt—Mn films to attain a high HUA and a high TB simultaneously, as well as desirable corrosion resistance.
For example, in the prior art spin-valve sensor with a Ni—Mn pinning layer, a Mn content of more than 57 at % is selected in order to attain a high HUA beyond 600 Oe. However, previous studies indicate that such a high Mn content leads to a low TB and to a low corrosion resistance. Hence, to operate a smaller sensor robustly at high temperatures for magnetic recording at ever increasing densities, very robust pinning layers must be found.
In previous studies, a spin-valve sensor with pinning layers formed of two antiferromagnetic films selected from two different binary alloy systems, such as Ir—Mn/Ni—Mn, Ir—Mn/Pt—Mn, Pt—Mn/Ni—Mn and Ni—Pt—Mn films, has been explored. The Ir—Mn film has been selected since it does not require annealing for developing exchange coupling with the Co—Fe film, thereby eliminating concerns on the Mn diffusion. The Pt—Mn film is generally preferred to be in contact with the Co—Fe film to minimize the Mn diffusion and to provide a high HUA. The Ni—Mn film is preferred not to be in contact with the Co—Fe film, while still providing a high TB. Nevertheless, since the Ir—Mn, Ni—Mn, and Pt—Mn films have different lattice parameters, the lattice mismatch causes exchange decoupling between the two different antiferromagnetic films, leading to difficulties in achieving the desired improvements.
In other previous studies, a spin-valve sensor with a pinning layer formed of an antiferromagnetic film selected from a ternary alloy system, such as Ni—Pt—Mn, Ni—Ir—Mn, Pt—Ir—Mn, etc., has also been explored. However, its antiferromagnetism has been found to be very weak, probably due to incompatibility of Ni, Pt, and Ir elements.
From the above discussion, it can be seen that it would be beneficial to further improve current spin-valve sensors through the discovery of more robust pinning layers that facilitate magnetic recording at increased densities.